Sound waves levitate and mix floating drops of liquid

Streams of sound are now all you need to make objects dance in the air and combine. A levitation device is the first to use high-frequency sound waves to bring together floating particles and liquid droplets. In principle, the technique could even levitate a person or animal – although it's not strong enough yet.

For now, such hands-free control could be used to study chemical reactions in extreme environments, to move hazardous materials and to simulate the low-gravity environment of space. At 24 kilohertz, the waves are too high-pitched to be audible to humans – but can be heard by some animals, including cats, bats and mice.

Other levitation methods use magnets or electrical fields, making mag-lev trains – and even levitating frogs – possible. But in these cases, the levitated objects must have particular magnetic or electrical properties.

Acoustic levitation imposes no such constraints. It can, in principle, float anything, says Dimos Poulikakos of the Swiss Federal Institute of Technology (ETH) in Zurich, Switzerland.

Levitation node

A sound wave is a pressure wave that produces a force and so has the potential to counteract gravity. To float things using sound, you need to ensure the force remains constant in a particular point in space.

This has been done previously, by using speakers or other resonators to fire pressure waves upwards and bounce them off a reflector. The original waves and their reflections then combine to create a standing wave, with a series of stationary "nodes" that stay put even as the wave oscillates.

If the standing wave has the right frequency, the force at these nodes exactly cancels gravity – and anything trapped there hovers in place.

Poulikakos and his colleagues wanted to go one step further and move and combine suspended objects. They built a system of computer-controlled resonators that creates a standing wave and that can vary its shape. As the wave shape changes, the nodes gradually move, carrying any trapped objects along with them (see graphic).

Coffee maker

The team used this system to make two objects react. In one instance they started with a particle of sodium suspended by one resonator, and a droplet of water suspended by another – and brought them together to produce a burst of energetic fizzing (see video). In another they used the system to collide a granule of instant coffee with a droplet of water.

"For the first time, you can move matter in a very controllable yet contactless manner," says Poulikakos. For now, the system can only levitate things that are as dense or less dense than water. (Sodium and coffee granules are less dense than water, which is why they float).

The system could be used to safely manipulate hazardous materials or to simulate microgravity experiments at a much lower cost. "From now on you don't have to go to space to do this, you can do it in your kitchen," says Poulikakos.

It might also be helpful for studying chemical reactions in new ways, such as seeing how liquids react when they are below their freezing points. This wouldn't work inside a container, because the cold surface would instantly cause a solid to form – but as long as a liquid doesn't come into contact with a surface, it's possible to keep it fluid even at very low temperatures.

Floating people

There is no intrinsic cap to the size of an object that this device can levitate. So it should be possible to float a person, as humans are only slightly denser than water. "I see no problem with that," says Poulikakos.

However, the larger the object, the bigger the amplitude of the sound waves required, so it might be a bumpy and dangerous ride: if a person slipped outside their node, they could take a pummelling because a large amplitude would mean a large upward or downward force. "Whether a human being could survive the acoustic forces, I'm not 100 per cent sure," says Poulikakos.

Bruce Drinkwater of the University of Bristol, UK, is impressed: "They've got some particular beautiful results," he says. He is excited by the potential for using the system to transport delicate objects, such as fragile electronics or biological cells.

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